1. Field of the Invention
The present invention relates to a semiconductor integrated circuit in which the resistance value of an internal terminator resistor can be measured.
2. Background Art
In recent years, as means for transmitting a number of signals, low voltage differential signaling (LVDS) interface circuits have been used which are semiconductor integrated circuits using current as signal transmitting means.
Such a LVDS circuit has a 100 Ω terminator resistor connected immediately before the input of a receiver circuit. The terminator resistor may be included in the semiconductor integrated circuit to reduce the number of components of the product. The terminator resistor has a resistance value defined by the predetermined specifications of the product. In the specifications, the resistance value is set at, e.g., 100 Ω±15 Ω.
In order to secure the resistance value in the product, it is necessary to measure a resistance value on a wafer in a die sorter test and measure a resistance value after the assembled product is put into a socket in the final test.
In a conventional method of measuring a resistance, both ends of a resistor are connected to a measuring device, a voltage is applied to the resistor, and a resistance is determined based on a value of passing current. In this measuring method, since a resistance is measured on two terminals, a measurement error may occur when there is a resistance between the measuring device and the resistor (for example, see Japanese Patent Laid-Open No. 2004-198168).
For example, a measurement on a wafer may cause a contact resistance between a pad and the probe of a measuring device and a measurement with a socket may cause a contact resistance between an assembled product and a pin of the socket. Therefore, even when the product is manufactured with a resistance value conforming to the specifications, the yields may decrease due to the problems on tests.
As a measuring method unaffected by a contact resistance, a method of measuring a resistance value on four terminals is available. In this measuring method on four terminals, a current is applied across first and second terminals connected to both ends of a measured resistor, and a potential difference between third and fourth terminals separately connected to both ends of the measured resistor is divided by the applied current. Thus the resistance value of the measured resistor is determined.
In the four-terminal method, current does not pass between the third and fourth terminals. Thus even when there is a contact resistance between the third and fourth terminals and the measured resistor, the resistance value of the measured resistor can be determined without being affected by the contact resistance.
However, the application of the four-terminal method in the aforementioned LVDS circuits requires two input/output terminals for testing in addition to two input/output terminals connected to a terminator resistor. Thus it is not possible to reduce the number of input/output terminals.